A man employed to guard a building, a site, or a piece of property, especially at night. Think of a construction site after dark, or an old factory, with one person walking around with a torch and checking that the doors are locked. The full phrase 'night watchman' is very common, and often the word simply means that. It is an older-sounding word. Modern job advertisements say 'security guard' or 'security officer', partly because 'watchman' contains 'man' and the job is not only for men — but there is no settled replacement ('watchwoman' exists but is rare, and 'watchperson' sounds odd), which is exactly why the whole word has quietly given way to 'security guard'. You still meet 'watchman' constantly in older books, in the Bible, in history, and in Taiwan and Hong Kong English where it is a normal everyday word for the person at the gate of a building or a school. There is also one special use to be ready for: in cricket, a 'nightwatchman' is a weaker batsman sent in late in the day to protect the better players, which has nothing to do with guarding a building.